Fibre pulsed lasers are increasingly being adopted as the laser of choice in a number of industrial applications, such as micromachining, drilling and marking. In peak-power-driven applications, such as marking, it is essential to retain high peak powers (in excess of 2.5 to 5 kW) at high repetition rates in order to achieve faster character marking and increased throughput.
Conventional single-stage Q-switched lasers are very efficient in storing energy. However, they are characterised by variable average power and substantial peak-power drop as the repetition rate is increased. In most cases, the peak power can drop below the process (e.g. marking) threshold with an adverse effect on speed and throughput. Master oscillator power amplifier (MOPA) configurations, on the other hand, can offer more controllability over the pulse characteristics and power performance of the pulsed laser and extend the operation space of a marking unit to higher repetition rates offering increased marking speed. There is a requirement for pulsed lasers which maintain the peak power over a 5 kW level for repetition rates in excess of 200 kHz. The average power should be in excess of 10 W, the pulse energy lies in the 0.1-0.5 mJ range or higher, the pulse duration to be variable between 10 ns and 200 ns, while the peak power should remain substantially constant at about the 5 kW or 10 kW level for rep rates in the range 10 kHz to >200 kHz. Additional requirements are good beam quality such as can be provided by a low-moded or single-moded fibre laser.
At these intensity and peak-power levels, special care is needed in the pulsed system to avoid the onset of optical non-linearities and optical damage. In addition, under the resulting high-gain, high inversion operating conditions the active fibre is not subject to photodarkening effects as that will reduce the efficiency and lifetime of the pulsed system.
A number of different pulsed fibre laser configurations have been proposed and used in a stand-alone fashion or as part of a master-oscillator power amplifier (MOPA) configuration. Q-switched fibre lasers, in particular, are quite attractive because they can produce high peak powers and several mJ pulse energies in a relatively simple and stable configuration. One of the main drawbacks of stand-alone Q-switched lasers, which are intended to be used in an industrial application in a versatile manner in order to increase the application space, is that all the parameters of interest, such as the pulse repetition rate (PRR), energy, peak power and pulse width, are interrelated and cannot be controlled independently. In particular, the peak power reduces as the pulse repetition frequency increases.
A number of these performance issues can be resolved and the required high peak power performance can be extended in the high PRR regime by using a multiple-amplification-stage MOPA configuration.
In this case the pulsed seed can be either a low power Q-switched laser or a directly modulated semiconductor laser. The latter can be controlled directly and provides much more freedom in defining the pulse shape and pulse repetition frequency, as well as, it gives the possibility of changing them at will to better fulfil the application needs. In addition, it is based on the well-developed and extremely reliable semiconductor technology developed over the years for the telecommunications industry. Different parameters of the amplified pulse sequences are defined accurately by controlling the gain distribution along the amplification chain.
The local inversion in a fibre amplifier increases considerably before the arrival of a pulse towards the output end of the amplifier. Knowledge of the inversion distribution is very important in defining the photodarkening rate in case where a fibre prone to this performance-degrading effect is used. As the pulse propagates, it depletes the inversion and increases its intensity. The amplification process also results in significant pulse reshaping and front-end sharpening. This is extremely important in defining pulse width and peak-power and as a consequence defines the onset of various non-linearities such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). Above a certain energy level, all pulses reshape (sharpen) considerably and reduce their pulse width. This is due to the fact that the pulse acquires enough energy to start saturating the amplifier. It is known that under such conditions, energy is extracted primarily by the leading edge of the pulse resulting in pulse reshaping and distortion. Peak power increases nonlinearly with pulse energy and inevitably exceeds the SRS threshold, which is typically around 5 kW to 10 kW, depending upon the fibre design and pulse shape.
Another important effect that limits the output power of pulsed fibre lasers is the formation of giant pulses. These can catastrophically damage the optical components in the system. The effect is believed to be highly dependent upon the peak power and the spectral properties of the laser and believed to arise from stimulated Brillouin scattering (SBS). When the non-linear threshold is reached, forward going pulses are reflected. Giant pulses are observed, and these can catastrophically damage the amplifiers (and other devices) in pulse laser systems. Unfortunately, the effect is stochastic in nature, and by itself very unpredictable. A single variation in the instantaneous spectral properties of a seed laser (such as a laser diode) which narrows the linewidth can result in an SBS event, and trigger giant pulse formation and subsequent catastrophic damage. Such damage has been observed in lasers months after they have been installed in industrial processing equipment.
Fibre lasers are often pumped by laser diodes. These laser diodes can be damaged by undesired optical radiation propagating from the laser to the diodes. The effect is particularly severe in pulsed lasers because laser diodes are damaged by peak power rather than the energy of a pulse. Pulsed lasers have much higher peak powers than continuous wave lasers. Therefore the requirement to isolate the pumps from the laser is more stringent in a pulsed laser than a continuous wave laser.
A very important issue related to the long-term behavior of Yb3+ doped fiber lasers and amplifiers is the effect of photodarkening. The effect shows as a gradual increase of the fibre background loss with time, which reduces the output power and overall efficiency of the optical system. It is believed to be related to the optical activation of pre-existing fibre color centres, with absorption bands mainly in the UV spectral region. However, the tails of the absorption bands extend into the near-IR adversely affecting the optical performance. Photodarkening results in gradual degradation and is not known to result in catastrophic sudden fibre failures. Photodarkening rate and final level is shown to be dependent on the active fibre degree of inversion and, as a result, different amplified systems will show different degradation.
Many applications of optical fibres require the generation and transmission of optical signals having intensities at which the transparency of the optical fibre degrades with time. The effect is known as photo-darkening, which is a light-induced change in the absorption of glass. The increase in absorption is believed to be due to the formation or activation of color centers that strongly absorb light in the UV and visible part of the spectrum.
In the spectral domain, photodarkening shows as a sharp loss increase below a wavelength of approximately 800 nm. The tail of this strong absorption band extends well into the 1 micron to 1.5 micron region and affects adversely the losses at both the pump and signal wavelengths. This has a severe limiting effect on the performance and overall efficiency of fiber lasers and amplifiers operating in this wavelength regime.
In the time domain, photodarkening shows as a gradual pseudo-exponential decrease of the laser or amplifier output power to an asymptotic value. The final power drop and related time scale seems to depend on the fiber laser or amplifier operating conditions, most notably the pump and average inversion levels, as well as, the operating temperature. The output power drop could be compensated by the provision of additional pump sources and/or the increase of driving pump current. Both measures are highly undesirable since the former results in increased unit cost while the latter results in accelerated of the pump-unit ageing and increased catastrophic failure probability.
Optical fibre lasers and amplifiers often include rare-earth dopant which can lead to photo-darkening via multi-photon processes. The effect is seen in at least Tm3+, Yb3+, Ce3+, Pr3+, and Eu3+ doped silica glasses.
Photodarkening is problematic when optical fibres are used in the industrial material processing. The effect can degrade the transmission in fibres used to deliver laser radiation from lasers (such as frequency-doubled, -tripled rod lasers, disk lasers, and fibre lasers) to a work piece, It can also severely limit the amount of optical power that can be generated in a fibre laser or amplified by an optical amplifier.
Conventional methods to reduce photo-darkening in glass are to use silica with high hydroxyl (OH) content, so-called “wet silica”. This can be loaded with deuterium and irradiated with ultra-violet (UV) light. However these approaches are not well suited for fibre lasers because the OH will increase the background loss of the optical fibre.
There is a need for a pulsed laser that maintains its peak power over a wide range of repetition frequencies and in which non-linear effects are controlled.
There is a need for fibre lasers that are resistant to pump damage.
There is a need for fibre lasers that are resistant to catastrophic damage from giant pulse formation.
There is a need for a photo-darkening resistant optical fibre. There is a related need for a fibre laser and amplifier that is resistant to photo-darkening. By photo-darkening, it is meant any light-induced decrease in transmission of glass, whether temporary or permanent.